At East Ohmer Creek, 22 miles south of Petersburg, is a tree believed to be the largest left on Mitkof Island. U.S. Forest Service Fish Biologist Eric Castro said foresters estimate the tree, which grew on a once-rich floodplain, is around 600 years old.

"Those giant pumpkins are what used to grow in this type of environment," Castro said.

That tree stands in contrast to those that have grown around it over the past 60 years, which have reached four to eight inches in diameter - about a tenth what would once have been expected.

Why does the floodplain that grew some of the largest trees left o...